This invention relates to a driving circuit of an ultrasonic motor utilizing a piezoelectric device.
An ultrasonic motor applies a frequency voltage to a piezoelectric device, oscillates a flexible member to excite a progressive wave and drives frictionally a moving member brought into pressure contact with the oscillation member.
The driving principle of the ultrasonic motor using the piezoelectric device as the driving source in the present invention will be explained. FIG. 17 shows an example of the principle of generation of a progressive wave in an ultrasonic motor of a progressive wave system. Reference numeral 171 represents the piezoelectric device. As shown in the drawing, it is polarized equidistantly with a width b and the polarization directions adjacent to one another are mutually opposite. An electrode 172 is formed by applying a conductive material such as silver to each piezoelectric device by technique such as vacuum deposition or plating and these electrodes are connected by signal lines 173, 174 and are driven by different signal sources, respectively. A portion having a width C is disposed in the electrode group wired by the signal lines 173, 174. Therefore, the center distance between the electrodes interposing C is a. It will be hereby assumed that the portion having the width C may or may not have polarization and electrode. The mechanism of generation of the progressive wave will be explained on the basis of the drawing and signals described above. The bend oscillation wave consisting of the progressive wave and the backward wave (having the opposite traveling direction to the progressive wave) can be expressed as follows on the basis of the piezoelectric device portion .circle.1 in FIG. 17 as the reference: EQU A sin (.omega.t-Kx)+A sin (.omega.t+Kx) (1)
Here, the formula (1) represents a so-called "steady wave". In contrast, the bend oscillation wave due to the piezoelectric device .circle.2 can be expressed as follows: EQU B sin (.omega.t-K(x+a)+.phi.)+B sin (.omega.t+K(x+a)+.phi.)(2)
where K=.omega./.gamma.=2.pi./.lambda. PA1 .lambda.: one wave length PA1 .phi.: phase difference angle with respect to the wave due to the piezoelectric device .circle.1 .
If the following relation is established in the formula (2), ##EQU1## then, the formula (2) can be expressed as follows: EQU B sin (.omega.t-Kx+.alpha..pi.)+B sin (.omega.t+Kx+.beta..pi.)(4)
Therefore, the bend oscillation wave excited by .circle.1 and .circle.2 is the sum of the formula (1) and (4).
If the condition which permits the existence of only the progressive wave is considered from the expansion of the formula (4), it can be understood that such a condition is in the cases where .alpha. is an even-numbered multiple of 1/2 and .beta. is an odd-numbered multiple of 1/2.
From the formula (3), a and .phi. can be expressed as follows by the formulas of .alpha. and .beta.: ##EQU2## In other words, when (.alpha., .beta.)=(0, 1), (2, 3), a=.lambda./4, .phi.=.pi./2 and when (.alpha., .beta.)=(2, 1), a=3/4, .phi.=3/2, and when these a and o are satisfied simultaneously, there exists the progressive wave. For instance, if the case where a=3/4, b=.lambda./2 and .phi.=3/2 is considered, the formulas (1)+(2) become as follows: EQU A sin (.omega.t-Kx)+A sin (.omega.t+Kx)+B sin (.omega.t-Kx)-B sin (.omega.t+Kx) (6).
Here, if the amplitudes A and B of both signals outputted from the driving circuit are A=B, it can be understood that the formula (6) becomes 2Asin (.omega.t-Kx) and only the progressive wave component remains. To attain reverse driving, only the backward wave may be left. Therefore, .alpha. and .beta. in the formula (5) may be reversed so that .alpha. is an odd-numbered multiple of 1/2 and .beta. is an even-numbered multiple of 1/2. If consideration is made on the basis of .circle.1 in practice, the phase of the signal applied to .circle.2 may be deviated by 180.degree. in comparison with the case of the normal driving.
FIG. 18 shows the principle of the rotation of the ultrasonic motor by the progressive wave. In the drawing, reference numeral 181 represents a stator or in other words, an oscillation member. When the progressive wave occurs on the basis of the principle shown in FIG. 17, one point on the surface portion describes an elliptical orbit in a leftward direction so that a moving member portion represented by 182 moves in a direction opposite to the traveling direction of the progressive wave. Since the above is described in "Nikkei Mechanical" (Sept. 23, 1985) and the detailed description of how the point on the surface describes the elliptical orbit is also given in this reference, the explanation will be hereby omitted.
The amplitude of the frequency of the oscillation member attains maximum at the mechanical resonance frequency of an oscillation member (flexible member+piezoelectric device) and driving force sufficient to drive the moving member can first be extracted by applying this resonance frequency or a frequency voltage approximate to the former to the piezoelectric device. It has been customary to measure the resonance frequency of the oscillation member by use of an FFT analyzer or the like, to impress the frequency in match with this resonance frequency to the piezoelectric device and to drive the moving member.
The resonance frequency of an ultrasonic motor changes in accordance with an ambient temperature or with the temperature change resulting from exothermy of the oscillation member itself. Since the moving member is brought into pressure contact with the oscillation member, the resonance frequency changes with this pressing force, too. Furthermore, the resonance frequency changes with a frequency voltage. The relationship is shown in FIGS. 10 and 11.
If the frequency of the frequency voltage applied to the piezoelectric device is fixed as has been made in the prior art technique, the driving force drops drastically and the motor stops if the resonance frequency of the ultrasonic motor changes.